Naturally occurring hydrated metal aluminum silicates are called zeolites. The synthetic counterparts of these materials with which this invention is concerned have a composition which is similar to some of the natural zeolites and accordingly the term "zeolite" has been applied to these synthetic materials. Zeolites can be synthesized with significant differences in crystal lattice and to distinguish the various synthetic zeolites from each other, specific zeolites are identified by a number and/or a letter designation following the term "zeolite." The two zeolites with which the present invention is specifically concerned are zeolite X and zeolite Y.
Zeolites selectively adsorb molecules on the basis of the size and shape of the adsorbate molecule and thus, they are called molecular sieves. Molecular sieves have a sorption area available on the inside of a large number of uniformly sized cages of molecular dimensions. With such an arrangement, molecules of a certain size and shape enter the cages and are adsorbed while larger or differently shaped molecules are excluded.
Zeolite X and zeolite Y are basically three-dimensional frameworks of SiO.sub.4 and AlO.sub.4 tetrahedrons cross-linked by the sharing of oxygen atoms. The electrovalence of each tetrahedron containing aluminum is balanced by the presence in the aluminosilicate framework of a cation, which in the case of synthetic zeolites is generally an alkali metal ion. Void spaces in the framework can be generated by evaporation of water molecules from the material as formed.
As pointed out in the prior art, for example U.S. Pat. No. 3,140,249 and U.S. Pat. No. 3,140,251, synthetic aluminosilicates of the type contemplated for use in this invention have been employed in hydrocarbon conversion processes. Molecular sieves containing alkali metal ions and having water molecules in the crystalline framework are not suitable, as such, for these uses. The water molecules reduce the adsorption capabilities of the crystals. Dehydration effects the loss of water of hydration and results in a crystal interlaced with channels of molecular dimensions that offer very high surface areas for the adsorption of foreign molecules.
Alkali metal ions generally result in catalysts of poor activity and thermal and/or hydrothermal stability. It is known that the alkali metal content of synthetic zeolites can be reduced by an ion exchange process wherein the alkali metal ions are replaced by various other metal ions which do not adversely effect catalyst life. Frequently rare earth metals are used to replace the alkali metal ions since the resultant rare earth exchanged zeolites are highly active for a wide variety of chemical reactions and at the same time possess the desired degree of longevity not found in the zeolites having a high alkali metal content.
The rare earth metal salts which are employed for this purpose may be either single rare earth salts of mixtures of salts such as rare earth chlorides or didymium chlorides. As hereinafter referred to, a rare earth chloride solution is a mixture of rare earth chlorides consisting essentially of the chlorides of lanthanum, cerium, neodymium and praseodymium with minor amounts of samarium, gadolinium and ytterbium. Rare earth metal chloride solutions are commercially available and generally contain the chlorides of a rare earth metal mixture in the following proportions, measured in terms of the oxides on a weight basis: cerium (as CeO.sub.2), 48%; lanthanum (as La.sub.2 O.sub.3), 24%; praseodymium (as Pr.sub.6 O.sub.11), 5%; neodymium (as Nd.sub.2 O.sub.3), 17%; samarium (as Sm.sub.2 O.sub.3), 3%; gadolinium (as Gd.sub.2 O.sub.3), 2%; ytterbium (as Y.sub.2 O.sub.3), 0.2%; and other rare earth oxides equal to 0.8%. Didymium chloride is also a mixture of rare earth chlorides, but one which has a low cerium content. It consists of the following rare earths determined as oxides and expressed on a weight basis: lanthanum, 45-46%; cerium, 1-2%; praseodymium, 9-10%; neodymium, 32-33%; samarium, 5-6%; gadolinium, 3-4%; ytterbium, 0.4%; and other rare earths 1-2%. Other mixtures of rare earths are equally applicable in effecting the exchange process for the reduction of the alkali metal content of the zeolitic material.
The two zeolites with which this invention is concerned have found widespread acceptance as catalysts in the petroleum conversion field. Both zeolites can be represented by a chemical formula, expressed in terms of moles of oxides. This formula is as follows: EQU 0.9 + 0.2 Na.sub.2 O: Al.sub.2 O.sub.3 : wSiO.sub.2 : xH.sub.2 O
wherein the value of w is between 3 and 6 in the case of zeolite Y, and between 2 and 3 in the case of zeolite X, and x may have a value up to about 9.
In producing the two zeolitic materials an aqueous reaction mixture containing oxides or materials whose chemical compositions can be represented as mixtures of oxides (i.e. Na.sub.2 O, Al.sub.2 O.sub.3, SiO.sub.2, and H.sub.2 O) is reacted suitably at a temperature of about 100.degree. C. for periods of time ranging up to 90 hours or longer. The product which crystallizes from the reaction mixture is filtered off, washed with distilled water until the effluent wash water in equilibrium with the zeolite has a pH of from about 9 to 12. The material after activation through drying is ready for use as a molecular sieve. By virtue of the fact that the conveniently available sources of oxides generally contain sodium, zeolite X and zeolite Y as produced, always have sodium cations which must be replaced with other cations for the reasons which have been outlined hereinabove. The replacement is generally accomplished by an exchange process whereby other cations are substituted in the crystal lattice for the sodium ions. The substitute cations should be ones which do not alter the crystalline framework. The combination of cations and specific framework is responsible for the great catalytic activity and selectivity of molecular sieves, and which impart a catalytic activity not found in the sodium forms of zeolite X and zeolite Y.
Several approaches have been employed for the production of exchanged zeolites. One of the early approaches is disclosed in U.S. Pat. No. 2,882,244. Therein it was suggested that the zeolite material be exchanged by treatment with an acid or a salt of another material having a cation which is capable of imparting desired physical properties to zeolites. Among the acids named were hydrochloric acid, which results in the replacement of the sodium ions with hydrogen ions. Other reagents disclosed in the patent were salts of any of many metals and ammonium salts. Among the metals suggested were the alkaline earth metals, nickel, zinc, silver and strontium. Measurements of the several ion exchanged forms showed that the replacement of sodium ions in the original sodium form of the zeolite was accompanied by a change in the size of the pores in the crystalline structure. This change effected the selectivity of the zeolitic materials. The magnitude of the effect varied with the nature of the cation.
Another method for removing the sodium cations from zeolites is based upon the concept of "decationization." In this approach, the metallic cations of the zeolitic molecular sieve are replaced with hydrogen or ammonium cations and the resultant exchanged material is heated to a temperature between about 350.degree. C. and about 600.degree. C. The decationization process is accompanied by the evolution of water. The water is believed to be constituted of hydrogen from the cation sites and an equivalent amount of oxygen released from the aluminosilicate framework. When a zeolitic material having a silica to alumina ratio less than 3 is subjected to the decationization process, the crystal framework is not capable of withstanding the removal of the cation and consequently collapses. Decationization is thus useful only for zeolites having a silica to alumina ratio of greater than 3. The decationized product provides an improved version of the sodium zeolites but is not fully satisfactory in comparison with certain forms of zeolites. Moreover, decationization results in a change in the crystalline structure of the zeolite. Though the change is normally slight, any deviation in structure from the original starting material is not desirable in view of the importance of the crystalline structure to the selectivity of the molecular sieve.
One attempt at getting the benefits of decationization without the adverse effect thereof is reported in U.S. Pat. No. 3,293,192 (Maher et al). In accordance with this patent, the composition of the zeolite which is subjected to decationization has a great impact upon the success of the process. The patentee starts with zeolite 14 HS sodium, which is said to be capable of withstanding the rigors of decationization in a uniquely successful manner. The Maher et al. process involves treating zeolite 14 HS sodium with an ammonium salt, amine salt or other salt which on calcination decomposes to leave appreciable portions of the zeolite in the hydrogen form. While the resultant exchanged zeolite is said to be ultrastable, the material does have a unit cell size which is smaller than the size of the unexchanged zeolite starting material. Moreover, the ionic nature and consequently the polar adsorption sites are modified during the exchange process.
Still another approach at removing sodium ions from zeolites to thereby overcome one of the principal causes of the lack of structural stability at high temperatures was reported in U.S. Pat. No. 3,140,252 (Frilette), and U.S. Pat. No. 3,140,253 (Plank). These patents are based upon the discovery that zeolites which are exchanged with aqueous solutions of rare earth salts possess the ultimate degree of structural stability and catalytic activity and selectivity desired for molecular sieves since the rare earth cations tend to impart stability to the aluminosilicate compositions to a far greater degree than do other cations, nevertheless the rare earth exchanged zeolites and highly active for a wide variety of chemical processes and thus these materials are more widely used than any other zeolites in the petroleum conversion industry.
There are several important considerations in choosing the exchange process by which sodium ions will be removed from the crystalline structure of aluminosilicates. It is important that the exchange process not affect the spatial arrangement of the atoms in the crystalline lattic, since the arrangement of the atoms contributes in large part to the desired activity of the catalyst. It is also important that the exchange process have only minimal effect upon the dimensions of the cages in the crystalline lattice, since these dimensions are critical to the retention of selectivity, one of the most important characteristics of molecular sieves. Likewise, the cation which is used to replace sodium in the aluminosilicate crystalline structure must impart good structural properties to the crystals and at the same time not adversely affect the catalytic activity thereof. The choice of exchange process and the cation used therein will also depend upon the particular zeolite which is to be treated, since the behavior of the zeolite to the exchange process is in large part dependent upon the particular composition of the zeolitic material.
In connection with the effect of the zeolite composition upon the behavior of the zeolite to the exchange medium, it is noted that zeolite X and zeolite Y lose their sodium cations at entirely different rates which are inherently determined by the nature of the respective materials. Zeolite X can be substantially fully exchanged with rare earth ions by subjecting the zeolitic material to a sufficient number of exchanges. The rate of replacement of the sodium ions decreases as the sodium oxide content of the zeolite X becomes lower but the rate remains sufficiently high for substantially complete removal of sodium ions to be effective after a sufficiently large number of exchanges and for long time periods. In the case of zeolite Y, it has not been possible to exchange all the sodium from the sodium form with rare earth cations, even with repeated exchange steps.
In view of the desirable characteristics of rare earth exchanged zeolite X and zeolite Y and the difficulty encountered in effecting the replacement of sodium ions with rare earth ions, it is an object of the present invention to provide a method for efficiently and economically replacing the sodium content of sodium zeolites with rare earth ions. It is a further object of the present invention to provide cracking catalysts of improved catalytic activity and structural stability when rare earth exchanged. It is a further object of the present invention to provide a process for exchanging sodium ions for rare earth ions while leaving the spatial relationship of the atoms and the size of the crystal lattice essentially unchanged. It is a still further object of this invention to provide an exchange process which is useful for exchanging crystalline aluminosilicates deposited on a supporting matrix which has catalytic activity whereby the resulting catalyst is prepared conveniently and economically without any sacrifice of usefulness.